Thermionic energy converters (TECs) are heat engines that convert heat directly to electricity at very high temperatures, typically >1000° C. Electrons are thermionically emitted from a hot emitter and collected at a relatively cool collector, effectively playing the role of the working fluid. FIG. 1A shows the basic configuration for this effect. An emitter electrode 102 is heated (shown schematically by arrows 108). Electrons 110 emitted by emitter electrode 102 are collected by collector electrode 104. The resulting electrical potential difference V0 can provide power to a load 106.
The corresponding energy diagram is shown on FIG. 1B. Here EF,E and EF,C are the Fermi levels of the emitter and the collector, respectively. Evac,E and Evac,C are the vacuum levels of the emitter and the collector, respectively. φE and φC are the work functions of the emitter and the collector. V0 is the voltage difference between the two electrodes and −q is the electron charge. Heating the emitter provides a distribution of electron energies in the emitter, schematically shown by curve 120 on FIG. 1B. Electrons having energy higher than Evac,E can escape from the emitter and be collected by the collector electrode. The theoretical efficiency of a TEC can exceed that of other solid-state technologies, such as thermoelectric converters, and can even be competitive with state-of-the art mechanical heat engines, such as steam turbines or Stirling engines.
Thermionic energy conversion was proposed in 1915. In 1941, Soviet researchers Gurtovoy and Kovalenko made the first laboratory converter, and in 1957, Hernqvist and co-workers at RCA demonstrated a practical converter with an efficiency of several percent. Hatsopoulos described various types of thermionic converters in his doctoral thesis at the Massachusetts Institute of Technology in 1956 and a subsequent two-volume monograph. Also in 1956, Moss published a review paper on using thermionic diodes as energy converters in the UK.
Since then, hundreds of papers on thermionic energy conversion have been published in the scientific and engineering literature. Intensive development during the 1960s-1970s for space applications culminated in the TOPAZ-II, a 6 kW converter, which was flown in 1987 by the Soviet space program. TOPAZ-II could operate for many years at up to 10 percent efficiency and had an emitter-collector gap of the order of 100 microns.
Although it was established in the 1950s that TECs with micron-scale gaps (<10 μm) are theoretically superior to their macroscopic counterparts, it was not until the last few decades that the development of MEMS process techniques enabled their fabrication. King and colleagues at Sandia National Laboratories proposed that thermionic energy converters could be microfabricated using MEMS wafer bonding processes. The authors suggested that the remaining hurdle was the development of low work-function materials and processes that could be integrated into these converters, in order to allow operation at relatively low temperatures (800-1300K). Subsequent modeling and fabrication efforts focused on micro-dispenser emitters for use in micro-miniature thermionic converters. Zhang and colleagues from the University of Michigan also fabricated a microfabricated thermionic converter combined with a combustion heat source in 2003. Thick silicon dioxide layers were used for thermal isolation and operation at high combustion temperatures (1000° C.) with large temperature gradients (50-100K per 100 μm) was demonstrated. In these microfabricated implementations, parasitic heat loss from emitter to collector was a major problem, limiting the conversion efficiency to a small fraction of 0.5%. In fact, a US government study in 2001 concluded that an efficient microfabricated thermionic energy converter is implausible because “it would be extremely difficult to maintain, for any reasonable period of time, a temperature difference of nearly 1000 K between two surfaces held apart by a miniaturized spacer that is a few microns thick”.